Oval and inverse curve are two alignments that are used widely recently in the design of horizontal alignment of highway. The arithmetic of calculating these two kinds of alignment is of diversity because of the complicated connection among their curve elements. Calculating these two kinds of alignment based on expression of spline, not only unifies the mathematical expression of every curve element, but also simplifies the method of calculating.Examining the reasonability of parameters after they are chosen can avoid bringing unreasonable one into the process of calculating and can increase the efficiency of program.Calculating coordinates of the points of easement curve (spiral curve) based on integral can increase the accuracy of the calculating result. In addition, the method is beneficial to debug program by checking connection relationship of the connected points that are located in different curve element at the same time.